The disclosure generally relates to a frequency generating circuit and, more particularly, to a frequency generating circuit using a quartz crystal resonator as a band-pass filter.
A quartz crystal resonator is widely used in many frequency generating devices, such as the Pierce oscillator, due to its simple architecture and low cost merits. As is well known in related art, the noise level of the signal generated by the quartz crystal resonator can be reduced by increasing the current injected into the quartz crystal resonator. However, the increasing of the current injected into the quartz crystal resonator accelerates aging of the quartz crystal resonator, thereby reducing the reliability of the quartz crystal resonator. Namely, there is a trade-off between the reliability of the quartz crystal resonator and the noise level of the signal generated by the quartz crystal resonator. The above trade-off property limits the applicable fields of the quartz crystal resonator. For example, it is difficult to apply the quartz crystal resonator in many applications demanding low power consumption.
In addition, the pulling range (i.e., the frequency adjustable range or frequency tuning range) of the conventional frequency generating device is severely restricted by the parasitic capacitance of the quartz crystal resonator. Accordingly, the pulling range of the quartz crystal resonator is highly depending upon the material of the quartz crystal resonator. As a result, the conventional frequency generating device requires utilizing high-end quartz crystal resonators in order to meet the wide pulling range requirement. In this situation, the overall hardware cost of the conventional frequency generating device is inevitably increased.